1. Field of the Invention
This invention relates, to zoom lenses capable of changing a range of variable focal length suited to television cameras or video cameras and, more particularly, zoom lenses capable of changing a range of variable focal length in which a space of the relay lens unit is utilized to detachably set a range changing lens unit therein to shift the range of variable focal length of the entire lens system toward the longer focal length and whose back focal distance is long enough to arrange, for example, a three-color separation optical system therein, suited to be used in the imaging system of the color television camera, etc.
2. Description of the Related Art
From the past, there have been proposed a variety of zoom lenses capable of changing a range of variable focal length from the ordinary limit to another one of different focal length, or changeable range zoom lenses.
Of these, the photographic lens of the type having a range changing lens unit built therein as arranged to move into, or away from, a part of the relay lens which is stationary behind the zoom section, when the range of variable focal length is changed, or the so-called built-in extender type, has found its use in many television cameras. With this type, it is in an instant that the range of variable focal length can be extended, and moreover the back focal distance of the whole lens system is kept unchanged during the extending of the range. Another excellent feature is that the total length of the lens remains constant.
The utilization of the built-in extender method in zoom lenses is proposed in, for example, Japanese Laid-Open Patent Application No. Sho 51-81634 and Japanese Patent Publication No. Sho 58-33528.
Of these, the Japanese Patent Publication No. Sho 58-33528 (U.S. Pat. No. 4,240,697) provides a zoom lens constructed from a zoom section and a relay lens unit. In turn, the relay lens unit is constructed from a relay front unit of negative refractive power and a relay rear unit of positive refractive power to thereby create a space therebetween. This space is either left empty, or occupied by a thick glass. And the relay front unit, or if the thick glass is present, the relay front unit and the thick glass, is or are retracted out of the optical path, and the thus-formed vacant space is used in attaching the range changing lens unit, so that the range of variable focal length is changed.
This method, because of its amenability to secure the space for the range changing lens unit, gets advantages of easily correcting aberrations, while nevertheless permitting the total length of the lens, too, to be shortened with relative ease. Since the zoom lens of that publication, however, has its stop positioned in front of the compensator of the zoom section, because, in some optical arrangement, the exit pupil gets nearer when the range changing lens unit is removed, or when in the ordinary condition, the television camera using the three-color separation optical system will suffer from appearance of color shading. Another problem is that the effective diameter of the relay rear unit tends to increase.
From these reasons, in recent years, a method of bringing the stop to the front of the relay lens unit within a similar arrangement for changing the range of variable focal length, has been used in many zoom lenses. However, alternative problems arise that, as the stop approaches the relay lens unit, the stop drive mechanism and the mechanism for attaching or detaching the range changing lens unit get closer to each other to increase the possibility of occurrence of mechanical interference therebetween, and that it becomes very difficult to locate all the elements and hold a high degree of tolerance on their structural spacings.
Besides this, as the method of changing the range of variable focal length of the zoom lens, it has also been put into practice that the front part of the relay lens unit is made fixed, and the space between the front and rear parts of the relay lens unit, or, if the thick glass is present, a space created by moving that thick glass away from the optical path, is used in attaching the range changing lens unit.
This method, due to its features that the relay front unit remains stationary during the time when the range changing lens unit is attached, and that the operating mechanism for the range changing lens unit can be assembled farther away from the operating mechanism for the stop by a distance equal to the axial thickness of the relay front unit, prevents both mechanisms from falling so close as to interfere with each other.
However, the use of this method in connection with the aim of increasing the back focal distance leads to a problem of increasing the diameter of the light beam in the relay lens unit which in turn calls for a great increase of the diameter of the range changing lens attachment unit to be used.
Further, in order to bring the exit pupil to a farther position, a thick glass must be used as inserted into the space between the relay front unit and the relay rear unit after the range changing lens has been detached, thereby producing another problem that the weight of the whole relay lens unit comes to increase largely.
As the zoom lens with the extending lens to be used in the space of the relay lens unit, mention may be made of U.S. Pat. Nos. 4,330,180, 4,318,592, 4,015,895 and 3,751,136.